Today is the Autumnal Equinox, the first day of fall in the northern hemisphere. The new season officially begins at 02:29 UTC, which is 10:29 pm Eastern Daylight Time.

The astronomical seasons are a product of the tilt of the Earth’s axis – a 23.5° angle – and the movement of the planet around the sun. Today, as fall begins, the Earth’s axis is tilted neither toward nor away from the sun.  This position distributes the sun’s energy equally between the northern and southern hemispheres.

Since the summer solstice in June, the arc of the sun’s daily passage across our sky has been moving southward and daylight hours have been decreasing. Today, it crossed the equator and we have approximately equal hours of day and night. The word “equinox” is derived from Latin and means “equal night”.

As a transitional season, autumn is a time when the heat of summer fades away and the chill of winter gradually returns. The largest drop in average temperature, however, usually lags the equinox by a few weeks.

Earth’s solstices and equinoxes. Image Credit: NASA

The Earth’s axis is tilted neither toward nor away from the sun on the Equinox.                   Image Credit: NASA